Fluid pressure device of the axial plunger type

ABSTRACT

A fluid pressure device of the axial plunger type including a housing, a rotatable shaft within the housing, a rotatable cylinder block operatively connected to the shaft and having a plurality of axial bores, plungers slidably mounted, respectively, within the bores, a valve plate located in abutment with an end surface of the cylinder block and having inlet and outlet ports which communicate successively with the bores as the cylinder block rotates, a swash plate pivotally supported upon the housing and operatively connected to the plungers for causing the plungers to reciprocate, and plunger shoes positioned between the plungers and the swash plate, the shoes being made of either a high tensile aluminum alloy, titanium, or a high tensile titanium alloy and comprising a slidable bearing surface made of a high load-bearing alloy which is adapted to bear against a planar surface of the swash plate.

llite States Patent 1191 Miyao et a1.

[4 Feb. 18, 1975 [54] 558 32 32 35? DEVIQE OF THE AXIAL FOREIGN PATENTSOR APPLICATIONS 716,871 8/1965 Canada 91/472 [75] Inventors: TakayukiMiyao; Hiroaki Maeda, 497,775 12/1948 Canada both of Toyota; MasanoriSate, 646,228 8/1962 Canada Nagoya; Hiroshi Moloyama, Kariya 1,251,2001/1961 France 91/498 all of Japan Primary Examiner-William L. Freeh [73]Asslgnee' s jf g Kalsha Attorney, Agent, or FirmOblon. Fisher, Spivak,

y p McClelland & Maier [22] Filed: Dec. 27, 1972 [21] Appl. No.: 319,031[57] ABSTRACT A fluid pressure device of the axial plunger type in- [30]Foreign Application Priority Data cluding a housing, a rotatable shaftwithin the housing, Dear/1971 Japan 46 3539 a rotatable cylinder blockoperatively connected to the shaft and having a plurality of axialbores, plungers 52] US. Cl. 91/488 slidably mounted, rPSPectively,Within the bores- 11 [51] Int. Cl. F01b 13/04 Valve Plate located inabutment with an end Surface of [58] Field of Search 91/472-507; theCylinder block and having inlet and Outlet POrts 308/237, 241 whichcommunicate successively with the bores as the cylinder block rotates, aswash plate pivotally sup [56] References Cited ported upon the housingand operatively connected to UNITED STATES PATENTS the plungers forcausing the plungers to reciprocate, and plunger shoes positionedbetween the plungers 2,934,480 4/1960 Slomrn and h swash p the shoes i glnade of either a 9x488 high tensile aluminum alloy, titanium, or a hightensile 3280758 10/1966 Learning et 8| 91/499 titanium alloy andcomprising a slidable bearing sur- 3:280:758 10/1966 Leemig metal/ 131::11: 91/499 face made of a high load-bearing alloy which is 3,297,5521/1967 Gisser et all 303/337 adapted to bear against a planar surface ofthe swash 3,709,107 1/1973 Alger 1 91/499 plate. 3,726,189 4/1973 Rubery.1 91/507 3,768,378 10 1973 Adam et a1. 91 499 6 Claims, 13 Drawmg FIgures :57 1 30 5 1. 1 1 /l 3 0 31 W21 24 33 m 1 51 53 22\, 33

XX 'k\ 25 7 2s 47 46 4.8 1.9

m 2a 58 26 39 I 41 i r 52 FATENTEU FEB] 8i975 SHEET 1 BF 3 PATENTEU FEBI 8 i 75 SHEET 2 OF 3 FIGS PATENTEB FEB 81975 SHEET 3 OF 3 FIG. 10

FIG. 9

C c c w x w 3 Z mil? g M5 6 w 0 \ll 6 A FLUID PRESSURE DEVICE OF THEAXIAL PLUNGER TYPE BACKGROUND OF THE INVENTION The present inventionrelates generally to fluid pressure devices, and more particularly toplunger shoes for use in fluid pressure devices of the axial plungertype which are capable of being operated as either a pump or a motor.

Fluid pressure devices of the type mentioned are well known in the art.Such devices include a plurality of plungers slidably disposed,respectively, within a plurality of cylindrical bores of a cylinderblock and operatively engaged with an inclined planar surface of astationary swash plate so that relative movement between the plungersand the swash plate is accomplished upon operation of the device.

The devices further include a plurality of plunger shoes interposedbetween the plungers and the swash plate for absorbing the plungerthrust forces due to the relative movement between the plungers and theswash plate. More particularly, each plunger shoe has a spherical socketportion which pivotally supports a spherical outer end of the associatedplunger mounted therein. A sliding end surface of the plunger shoebearing against the planar surface of the swash plate has a centralannular recess which is adapted to receive fluid pressure therebyproviding a hydraulic balancing force which also acts as a hydraulicbearing means for providing lubrication between the plunger shoe and theswash plate.

However, due to the fact that the sliding surface of the shoe will beexposed to high load and high rotational speed conditions, phenomena,such as for example, excessive wear and heating of the shoes slidingsurface will occur. These phenomena will of course substantiallyrestrict the operation of the device under such high pressure or highrotational speed conditions.

ln addition, it should also be noted that the center of gravity of eachplunger shoe does not correspond to the geometrical center of thespherical surface of each plunger. Thatis, the center ofgravity of theplunger shoe is displaced toward the planar surface of the swash plate.Thus the plunger shoe is subjected, at the center of the sphericalsurface thereof, to a moment due to centrifugal force which isproportional to the weight of the plunger shoe and the square of therotational speed of the device when the device is in its operationalstate. This results in the condition wherein the plunger shoe becomesinclined with respect to the planar surface of the swash plate.

Furthermore, the conventional plunger shoe is generally made of amaterial such as for example, phosphor bronze which has the necessarybearing ability, however, the specific gravity of phosphor bronze islarge and, therefore, the moment which tends to cause the plunger shoeto become inclined with respect to the swash plate becomescorrespondingly large. Consequently, the plunger shoe is urged at oneend thereof to contact the planar surface of the swash plate along alineal locus and consequently a satisfactory film of lubrication cannotdevelop between the plunger shoe and the swash plate planar surface, andfurther, metallic contact between the plunger shoe and the planarsurface can result. This of course hastens wear of the plunger shoe,which is generally made of a softer material than the swash plate, andcauses the plunger shoe to become excessively heated which furtherhastens the wear thereof. Thus, the effective service life of theplunger shoe will be substantially reduced.

Still further, although some amount of leakage between the relativesliding surfaces of the plunger shoe and the swash plate is desirablefor lubrication therebetween, leakage may become excessive due to theincreased inclination of the plunger shoe relative to the swash plate sothat the volume efficiency of the fluid pressure device will be reduced.lt is additionally understood that the various disadvantages mentionedheretofore will of course be increased during operation of the device athigh load and high speed conditions.

Also, while it is necessary that in order to prevent deformation of theplunger shoe that it has a higher yield strength, it is also requiredthat the shoe be malleable so that the shoe may be caulked to thespherical 'outer end of the associated plunger.

SUMMARY OF THE INVENTION Therefore, it is an object of the presentinvention to provide an improved fluid pressure device of the axialplunger type which does not exhibit the above mentioned disadvantages.

Another object of the present invention is to provide an improved fluidpressure device of the axial plunger type wherein each plunger shoe hasa high wear resistance and a high heat-resistance.

Still another object of the present invention is to provide an improvedfluid pressure device of the axial plunger type wherein the plunger shoeis made of a material such as a high tensile aluminum alloy, titianium,or a titanium alloy which is of relatively small specific gravity and ofhigh tensile strength, the plunger shoe comprising a sliding surfacemade of an alloy which exhibits high load-bearing properties.

Yet another object of the present invention is to provide an improvedfluid pressure device of the axial plunger type wherein the slidingsurface of the shoe is made of an aluminum alloy such as for example, analloy of aluminum (92-.96percent) and silicon (48percent) respectively,which exhibits excellent structural adaptability, high wear-resistance,and high heatresistance.

It is a further object of the present invention to provide an improvedfluid pressure device of the axial plunger type wherein the slidingsurface of the shoe is made of a material, such as for example, a copperalloy, which may be for example, phosphor bronze, high-lead phosphorbronze, or the like, or a copper alloy of sintered material whichexhibits excellent structural adaptability, high wear-resistance, andhigh heatresistance.

A still further object of the present invention is to provide animproved fluid pressure device of the axial plunger type wherein theplunger shoe further comprises a spherical socket portion for receivablymounting the spherical outer end portion of the associated plunger, thespherical socket portion including a film made of an alloy of aluminumand silicon, a copper alloy or a copper alloy of sintered material whichexhibits excellent structural adaptability and a high wearresistance.

BRIEF DESCRIPTION OF THE DRAWINGS Various other objects, features, andattendant advantages of the present invention will be more fullyappreciated as the same becomes better understood from the followingdetailed description when considered in connection with the accompanyingdrawings in which like reference characters designate like orcorresponding parts throughout the several views, and wherein:

FIG. 1 is a cross sectional view of a fluid pressure device of the axialplunger type representing one embodiment of the present invention;

FIG. 2 is an enlarged cross sectional view of a plunger shoe for usewithin the fluid pressure device of FIG. 1;

FIG. 3 is an enlarged view of designated portion A of FIG. 2;

FIG. 4 is an explanatory view disclosing the function of the presentinvention;

FIG. 5 is a view similar to that of FIG. 2, disclosing however, a secondembodiment of the plunger shoe;

FIG. 6 is an enlarged view of designated portion B of FIG. 5;

FIG. 7 is a view similar to that of FIG. 2, disclosing however, a thirdembodiment of the plunger shoe;

FIG. 8 is an enlarged view of designated portion C of FIG. 7;

FIG. 9 is an enlarged view of designated portion D of FIG. 7;

FIG. 10 is a view similar to that of FIG. 2, disclosing however, afourth embodiment of the plunger shoe;

FIG. 11 is an enlarged view of designated portion E of FIG. 10;

FIG. 12 is an enlarged view of designated portion F of FIG. 10; and IFIG. 13 is a view similar to that of FIG. 2, disclosing however, a fifthembodiment of the plunger shoe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to thedrawings and more particularly to FIGS.. 1,2, 3 and 4 thereof, ahydraulic fluid device of the axial plunger type includes a housinghaving two end plates 21 and 22 respectively secured thereto by screw orbolt means 23 and 24. An axial drive shaft 25 is rotatably supported atone end of the housing 20 by means of bearings 26 and extends through arotatable cylinder block 27 to the end plate 21 wherein the drive shaft25 is rotatably supported by means of bearings 28, the bearings 26preventing the drive shaft 25 from experiencing any axial movement. Thedrive shaft 25 is operatively and drivingly connected to the cylinderblock 27 through a plurality of connecting pins 29 circumferentiallypositioned around the drive shaft 25 whereby the cylinder block 27 isadapted to rotate together with the drive shaft 25.

The cylinder block 27 contains a plurality of axially aligned,circumferentially positioned longitudinal bores 30 in which one of aplurality of plungers 31 is respectively slidably mounted so as todefine therein a hydraulic piston chamber 32. Although only two plungers31 are shown in FIG. 1, there may be for example, nine plungers 31, asis known in the art. The outer ends 33 of the plungers 31 are ofspherical configuration and are universally connected to and pivotallymounted within plunger shoes 34, that is, the shoes 34 have sphericalsocket portions for receiving the outer ends 33 thereby serving as ballbearings.

As viewed in FIGS. 2 and 3, each of the plunger shoes 34 includes a bodyportion 35 made of a high tensile aluminum alloy, titanium, or a hightensile titanium alloy, which is processed with grid blast at an endsurface '36 thereof. Upon the end surface 36 there is applied a layer ofmolybdenum .37 upon which a high loadbearing alloy 38, such as forexample, an alloy of aluminum and silicon or phosphor bronze is furtherapplied. The application of the molybdenum layer 37 is effective tofirmly secure the alloy layer 38 thereon. The alloy of aluminum andsilicon comprises respective proportions of aluminum and silicon of92-96z4-8 and is subsequently subjected to a forging process. Theresulting alloy of aluminum and silicon is thus excellently adapted forthe high-load and high-speed conditions of the device A sliding endsurface 39 of the shoe 34 engages a sliding planar surface 40 of a swashplate 41 for the purpose of facilitating reciprocation of the plungers31. A disk-like retainer 42 has a plurality of holes in which theplunger shoes 34 are positioned and a-retaining member 43 is secured tothe swash plate 41 by means of bolts, not shown, in order to secure theretainer 42. The shoes 34 are thus maintained in slidable contact withthe surface 40 of the swash plate 41.

The swash plate 41 is supported within the housing 20 by means oftrunnions, not shown, which are secured to the housing 20 in a wellknown manner, the swash plate 41 therefore being adapted to be pivotedabout an axis transverse to the axis of the drive shaft 25. The lowerend 44 of the swash plate 41 is mechanically connected to conventionalactuating means, not shown, through a connecting link 45 so that theinclination of the swash plate 41 may be automatically or manuallycontrolled in order to adjust the displacement of the plungers 31.

A coil spring 46 disposed about and anchored to the drive shaft 25through an annular collar 47 biases another annular collar 48 against asnap ring 49 which is secured to the cylinder block 27, and thus, biasesthe cylinder block 27 and a valve plate 50 against the end plate 21, thevalve plate 50 and the cylinder block 27 thereby being urged intoface-to-face contact. The valve plate 50 is mounted against the endplate 21 by pin means 51 so that the valve plate 50 is retained againstrotation with the cylinder block 27 and is formed with a pair of ports52 and 53 of the conventional elongate arc-shaped type which arehydraulically connected, respectively, to associated hydraulic systems,not shown. A series of axially extending passages 54 is formed,respectively, within the cylinder block 27 and are successively broughtinto association with the ports 52 and 53 in a known manner duringrotation of the cylinder block 27 so that the hydraulic fluid may betransmitted to and from the hydraulic chambers 32.

As best shown in FIG. 4, the surface 39 of each plunger shoe 34 has acentral annular recess 55 while an axially extending passage 56 providedwithin each plunger 31 is hydraulically connected to the recess 55through an orifice 57 formed within the shoe 34. The recess 55 will thusreceive hydraulic fluid when the associated chamber 32 is under pressureand will serve as a hydraulic bearing means acting between the surfaces39 and 40. The hydraulic fluid pressure within the recess 55 willproduce a thrust upon the shoe 34 which will tend to oppose the plungerthrust, the thrust produced by the fluid pressure within the recess 55depending upon the leakage flow between the surfaces 39 and 40 and thepressure drop occurring at the orifice 57. As the gap between thesurfaces 39 and 40 increases there will be greater leakage flowtherebetween which will in turn cause a greater reduction of fluidpressure within the recess 55 and vice versa. Thus the fluid pressurewithin the recess 55 is balanced thereby providing a constant gapbetween the surfaces 39 and 40. An annular sealing member 58 is mountedwithin, and disposed about, end plate 22 and drive shaft 25respectively.

Referring'now to FIGS. 5 and 6, the plunger shoe 34a includes a bodyportion 35a made of a high tensile aluminum alloy, titanium, or a hightensile titanium alloy, and an end surface 36a of the body 35a which isprocessed with gridblast and thereafter covered with a high load-bearingalloy 38a, such as for example, an alloy of aluminum and silicon orphosphor bronze. Of course, if it is desired, the high load-bearingalloy 38a may be secured to the end surface 36a by any one of theconventional methods such as for example, friction welding, plating orpowder metallurgy. In the instance that the method of plating isutilized, the surface to be plated is initially coated with zinc and isthereafter plated.

In FIGS. 7, 8 and 9, a spherical surface 59 of the shoe 34b adapted toreceive therein the spherical outer end portion of the associatedplunger is processed with gridblast and is thereafter covered with ahigh loadbearing alloy 60, such as for example, an alloy of aluminum andsilicon or phosphor bronze. The remaining structure of shoe 34b issimilar to that shoe disclosed within the embodiment of FIGS. 5 and 6.

Referring now to FIGS. 10, 11, and 12, a spherical surface 590 of theshoe 34c initially processed with gridblast is subsequently covered witha layer of molyb denum 61 and is thereafter covered with a highloadbearing alloy 62, such as for example, an .alloy of aluminum andsilicon or phosphor bronze. The remaining structure of the shoe 34c isthe same as that shoe disclosed within the embodiment of FIGS. 2 and 3.

In FIG. 13, a base portion 35d of the shoe 34d is made of a high tensilealuminum alloy, titanium or a high tensile titanium alloy. Upon thesurface of the base portion 35d,' a powdered layer 63 of a highload-bearing alloy, such as for example, an alloy of aluminum andsilicon, phosphor bronze, or high lead phosphor bronze is applied and issubsequently treated by applying a proper amount of heat. The powder 63thus applied has a porous surface 64 so that even if the shoe makesmetallic contact with the sliding surface of the swash plate, oilcontained within the porous surface will ooze out therefrom due to theheat generated therebetween so as to thereby provide good lubrication.The spherical socket portion of the shoe 34d for receiving the plungeris similarly covered with a powder metallurgy material 65 of a highload-bearing alloy whereby the same effect mentioned above can beobtained.

In operation, although this device may function as either a pump or amotor, its operation as a pump will now be described, in which case, thedrive shaft 25 is connected to a suitable prime mover, not shown. Thedrive'shaft 25 is rotated by the prime mover and in turn will causerotation of the cylinder block 27. Therefore, the chambers 32 willsequentially communicate with the low pressure or inlet port 52 of thevalve plate 50 and then with the high pressure or outlet port 53 of thevalve plate 50 in a conventional manner. At the same time, the plungers31 and the shoes 34 will be rotated in a circular path around the axisof the drive shaft 25 during rotation of the cylinder block 27, therotary motion of the plungers 31 and shoes 34 being confined to a locusdefined by the inclined position of the swash plate 41 whereby theplungers 31 will be reciprocated within the bores 30. Thus, hydraulicfluid will enter the chambers 32 at the inlet port 52 for low pressurewhen the plungers 31 are movedon their suction strokes, and will bedelivered from the chambers 32 under pressure at the outlet port 53 forhigh pressure when the plungers 31 are moved on their discharge strokes,a pumping action thereby being accomplished.

In operation of the device as a motor, the inlet and outlet ports arereversed and, therefore, the fluid valved through the port 53 of thehigh pressure side or inlet side of the valve plate 50 enters thechambers 32 through the passages 54. The fluid pressure within thechambers 32 acts upon the plungers 31 which in turn urge the plungershoes 34 against the surface 40 of the swash plate 41 so as to effect asliding motion of the shoes 34 upon the surface 40, this actionresulting in rotation of the cylinder block 27 and the drive shaft 25connected thereto. The hydraulic fluid will thus be transmitted to thechambers 32 from the inlet port 53 for high pressure when the plungers31 are moved on their suction strokes and will then be delivered fromthe chambers 32 at the outlet port 52 for low pressure when the plungers31 are moved on their discharged strokes, a motor action thereby beingaccomplished.

In either case, the fluid within the chamber 32 is transmitted to therecess 55 so that the hydraulic balancing force between the slidingsurfaces of the shoe 34 and the swash plate 41 will be provided, as wasnoted hereinbefore.

In FIG. 4 G represents the center of gravity of shoe 34 and F representsthe centrifugal force exerted at the center of gravity G of shoe 34during operation of the device. The centrifugal force F increases inproportion to the weight of the shoe and the square of the rotationalspeed of the device. As shown in FIG. 4, the center of gravity G doesnot correspond to the geometrical center of the spherical socket surfaceof the shoe 34 and is biased or displaced toward the sliding surface 40of the swash plate 41 so that a moment M will be produced, the moment Mcausing the shoe 34 to become inclined with respect to the slidingsurface 40. Therefore, a reaction force R is exerted upon the shoe so asto balance the moment M. As a result, the shoe 34 will be inclined at anangle 0.

Now assuming that devices constructed identically will be actuated underthe same conditions, the centrifugal force F exerted upon the shoe 34becomes smaller for a smaller weight of the shoe 34, andcorrespondingly, the moment M which causes the inclination of the shoe34 becomes smaller. Similarly, the reaction force R becomes smaller and,thence, the angle of inclination 0 of the shoe 34 with respect to thesliding surface 40 becomes smaller. Accordingly, it is desired tominimize as much as possible the :specific gravity of the shoe, andtherefore, the base portion of the shoe according to the presentinvention is made of a high tensile aluminum alloy, titanium, or a hightensile titanium alloy, and it is desirable that the rate of use of thehigh load-bearing alloy as a bearing material will be minimized as muchas possible. The best results may be obtained when the rate of use ofthe high load-bearing alloy, that is to say, the thickness thereof, isabout 0.1 to 0.5 mm.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is to be understood,therefore, that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A fluid pressure device of the axial plunger type comprising:

a housing;

a rotatable shaft within said housing;

a rotatable cylinder block operatively connected to said shaft andprovided with a plurality of longitudinal cylindrical bores therein;

plungers slidably mounted, respectively, within said bores and includingspherical outer end portions;

a valve plate located in abutment with an end surface of said cylinderblock and having a pair of ports which communicate successively withsaid bores as said cylinder block rotates;

a swash plate pivotally supported upon said housing and having aslidable planar surface; and

a plurality of plunger shoes associated with said spherical outer endportions of said plungers at spherical socket portions thereof andincluding slidable bearing surfaces adapted for engaging said slidableplanar surface of said swash plate for causing said plungers toreciprocate,

said shoes being formed of a light weight high tensile metal materialand said slidable bearing surfaces of said shoes being formed of a highload-bearing alloy, wherein said high-load bearing alloy is applied tosaid slidable bearing surfaces of said shoe as a powder metallurgymaterial.

2. A fluid pressure device of the axial plunger type comprising:

a housing;

a rotatable shaft within said housing;

a rotatable cylinder block operatively connected to said shaft andprovided with a plurality of longitudinal cylindrical bores therein;

plungers slidably mounted respectively, within said bores and includingspherical outer end portions;

a valve plate located in abutment with an end surface of said cylinderblock and having a pair of ports which communicate successively withsaid bores as said cylinder block rotates;

a swash platepivotally supported upon said housing and having a slidableplanar surface; and

a plurality of plunger shoes associated with said spherical outer endportions of said plungers at spherical socket portions thereof andincluding slidable bearing surfaces adapted for engaging said slidableplanar surface of said swash plate for causing said plungers toreciprocate,

said shoes being formed of a light weight high tensile metal materialand said slidable bearing surfaces of said shoes being formed of a highload-bearing alloy of aluminum and silicon.

3. A device as set forth in claim 2, wherein said alloy of aluminum andsilicon contains respective amounts of aluminum and silicon in theapproximate porportion of 92-96: 4-8.

4. A device as set forth in claim 2, where said slidable bearingsurfaces of said shoes are made either of a copper alloy or a copperalloy of sintered material.

5. A device as set forth in claim 4, wherein said copper alloy is eitherphosphor bronze or high-lead phosphor bronze.

6. A device as set forth in claim 5, wherein a layer of molybdenum isprovided between said copper alloy and said light weight high tensilemetal material.

1. A fluid pressure device of the axial plunger type comprising: ahousing; a rotatable shaft within said housing; a rotatable cylinderblock operatively connected to said shaft and provided with a pluralityof longitudinal cylindrical bores therein; plungers slidably mounted,respectively, within said bores and including spherical outer endportions; a valve plate located in abutment with an end surface of saidcylinder block and having a pair of ports which communicate successivelywith said bores as said cylinder block rotates; a swash plate pivotallysupported upon said housing and having a slidable planar surface; and aplurality of plunger shoes associated with said spherical outer endportions of said plungers at spherical socket portions thereof andincluding slidable bearing surfaces adapted for engaging said slidableplanar surface of said swash plate for causing said plungers toreciprocate, said shoes being formed of a light weight high tensilemetal material and said slidable bearing surfaces of said shoes beingformed of a high load-bearing alloy, wherein said high-load bearingalloy is applied to said slidable bearing surfaces of said shoe as apowder metallurgy material.
 2. A fluid pressure device of tHe axialplunger type comprising: a housing; a rotatable shaft within saidhousing; a rotatable cylinder block operatively connected to said shaftand provided with a plurality of longitudinal cylindrical bores therein;plungers slidably mounted respectively, within said bores and includingspherical outer end portions; a valve plate located in abutment with anend surface of said cylinder block and having a pair of ports whichcommunicate successively with said bores as said cylinder block rotates;a swash plate pivotally supported upon said housing and having aslidable planar surface; and a plurality of plunger shoes associatedwith said spherical outer end portions of said plungers at sphericalsocket portions thereof and including slidable bearing surfaces adaptedfor engaging said slidable planar surface of said swash plate forcausing said plungers to reciprocate, said shoes being formed of a lightweight high tensile metal material and said slidable bearing surfaces ofsaid shoes being formed of a high load-bearing alloy of aluminum andsilicon.
 3. A device as set forth in claim 2, wherein said alloy ofaluminum and silicon contains respective amounts of aluminum and siliconin the approximate porportion of 92- 96: 4-
 8. 4. A device as set forthin claim 2, where said slidable bearing surfaces of said shoes are madeeither of a copper alloy or a copper alloy of sintered material.
 5. Adevice as set forth in claim 4, wherein said copper alloy is eitherphosphor bronze or high-lead phosphor bronze.
 6. A device as set forthin claim 5, wherein a layer of molybdenum is provided between saidcopper alloy and said light weight high tensile metal material.